What is "tagged memory"?

Question

What is "tagged memory" and how does it help in reducing program size?

Answer 1

You may be referring to a tagged union, or more specifically a hardware implementation like the tagged architecture used in LISP machines. Basically a method for storing data with type information.

In a LISP machine, this was done in-memory by using a longer word length and using some of the extra bits to store type information. Handling and checking of tags was done implicitly in hardware.

For a type-safe C++ implementation, see boost:variant.

Answer 2

Not sure, but it is possible that you are referring to garbage collection, which is the process of automatically disposing of no longer used objects created when running a program.

"Tagged memory" can be a synonym for mark-and-sweep, which is the most basic way to implement garbage collection.

If this is all wrong, please edit your question to clarify.